This invention relates in general to direct tension indicating and maintaining washers and in particular to direct tension indicating and maintaining washers that maintain bolt tension and/or indicate when the proper bolt tension has been achieved.
The present invention is an improvement to commonly-assigned U.S. Pat. No. 5,931,618 issued Aug. 3, 1999 to Wallace et al. hereby incorporated by reference herein. Referring to FIGS. 10-13, the prior art device comprises a direct tension indicating washer 60 having bumps 12. As bolt 50 is tightened, the head of bolt 50 squeezes bumps 12 flat against surface 10. An indicating material 64, such as colored silicone compound, is deposited in indentations beneath bumps 12 (see FIG. 11). Thus, as bumps 12 become flattened, so do the indentations, causing the indicating material to squeeze out through channels 62 from beneath direct tension indicating washer 60 and the bolt head as shown in FIG. 13, providing an immediately visible indication that a desired bolt minimum tension has been reached.
There are, however, applications which a single known bolt tension is insufficient. For example, sometimes it is desirable to know that a bolt has be tensioned to within a specified range. In this circumstance, it would be desirable to know not just that a bolt has been tensioned beyond a specific minimum, but that it has not been tensioned too far, i.e., beyond a maximum allowable tension.
Additionally, there are some bolting applications where the bolt must be tightened to two specific tensions. For example, a lower tension may be needed for pre-assembly conditions and xe2x80x9crunning inxe2x80x9d of the machine or equipment, and then a higher tension for service life.
The limitations of the prior art noted above are overcome by providing a direct multi-tension indicating washer that extrudes indicating material of varying colors depending on the compression force applied to it. The direct multi-tension indicating washer has a first and second surface having respective bumps and indentations. The compression force on the washer is equal to the tension in the bolt which has been inserted through the washer. The indentations in the second surface are filled with a solid extrudable colored material. By making at least a first bump taller than at least a second bump, the first bump will compress before the second bump, causing it to extrude the color material in its corresponding indentation before the second bump does the same as the tension in the bolt is increased. By making the material under the first bump a different color than the material under the second bump, an operator can determine which of two desired tensions the bolt has reached.
In use, an operator merely tightens the bolt passing through the washer or a nut threaded onto the bolt until the bolt reaches a first desired tension as indicated by the extruded material of the first color, such as green. An intervening step is performed at the first bolt tension, if required. Then, the operator continues to tighten the bolt or nut until the bolt reaches a second desired tension as indicated by the extruded material of the second color, such as red.